The invention relates to a method and apparatus for making retro-reflective sheets having prismatic reflectors and more particularly to prismatic molds used for manufacturing prismatic reflective vinyl sheets where seam lines produced from edges between adjacent prismatic molds can be visually either eliminated or disguised.
It is known in the art to use very finely engraved molds and dies in a myriad of applications such as holographics, lenticular lenses, fresnel lenses, prismatic reflective materials, diffractive metallized products and the like. In many of these applications the molds are used to form these articles by pressing against a raw material such as vinyl and the molds require extremely fine and micro-detailed features.
Such micro-fine detail is especially required for the making of molds needed to produce prismatic reflective materials in vinyl sheets. Generally such micro-fine tooling is made with diamond turning equipment. This equipment allows very fine reflective structures to be created in the form of tiny 3-sided pyramids, which result in cube corner reflective elements once the mold is pressed into a vinyl sheet or if a liquid resin is cast onto the mold then solidified or radiation cured onto a vinyl polyester or other thermoplastic sheet. Tiny micro-pyramids of 4 or more sides, as well as with different sizes or shapes can also be made with this method.
Retro-reflective sheeting comprising of prismatic cube corner formations are well known in the industry. Rowland U.S. Pat. No. 3,811,983 describes one method for making such materials. Tooling for these cube corner structures is also referenced in this patent. The tooling described in this patent details the size of the cube corners as being 0.00235xe2x80x3 in depth and 0.0056xe2x80x3 on a side. This indicates that these are extremely fine structures, with a surface quantity of over 40,000 per square inch. Such fine structures are still widely employed and utilized today, but multiple molds with such fine structures cannot be joined, seamed, tiled or otherwise butted together without forming visibly apparent seam lines of some sort on the vinyl or other thermoplastic substrate.
In U.S. Pat. No. 5,171,624 prismatic molds are shown where some of the prisms have flat sides, and others are arcuate; some even have their axes tilted. While U.S. Pat. No. 5,657,162 deals with adding a reflective coating to reflective material, much reference is made to the prismatic reflective dies needed to make this material. In this case the dies shown have prisms of different sizes and angles.
But in the aforementioned patents, no matter what the prism size, the angles or the position of the axes of the prisms, there is a maximum size to which a micro-fine prismatic mold can be made. When another mold is joined to the first one to make a larger retro-reflective sheet, unwanted seam lines will appear on the mold, and then ultimately appear on the finished reflective sheet material.
U.S. Pat. No. 6,036,322 refers to similar retro-reflective structures with cube-corner reflective elements, and where the elements are oriented in different directions. What is taught here is how the elements can be oriented first in one direction and then in a second direction at 90xc2x0 to the first direction. It goes on to show that a third array can be oriented at 180xc2x0 to the first direction, and a fourth array at 270xc2x0 to the first direction. When looking at the patent drawings, this clearly shows seam lines between each array of prism directions, which seams are derived from making the tooling for these micro-prisms.
As used herein, prismatic retro-reflective articles include cube corner and prismatic as well as other shapes of retro-reflective elements formed into a plastic material made by pressing a die into it, or applying a liquid resin onto the mold, curing it to solidify it on a thermoplastic substrate.
There are angularity advantages by orienting the prisms in these different directions, but by doing this, more internal joint-lines than usual will appear across the face of the ultimate reflective product. In addition to this angling of prisms, when a first mold is joined to the next mold for manufacturing, a distinct seam line will appear between each mold.
With current technology, it is virtually impossible to eliminate unwanted seam lines in the manufacture of prismatic retro-reflective materials in large sheets. There are limitations on how big one can make a single mold in a diamond turning machine. Some machines can produce a continuous flawless pattern of micro-prisms at a size of 8xe2x80x3xc3x978xe2x80x3; some at 9xe2x80x3xc3x979xe2x80x3; some at 9xe2x80x3xc3x9710xe2x80x3 and some equipment possibly even slightly larger than this. However, in the manufacture of larger retro-reflective sheets or in the case of continuous roll-goods production, multiple molds will be needed and one or more visually disturbing seams would, therefore, become apparent. Also, the larger molds are expensive to make and thus increase the costs of the retro-reflective articles made with them.
As technology improves the mold plate sizes may even get larger. But even if the die plates can be made larger, when such molds are installed on manufacturing equipment, whether as flat plates or in rotary form, at some point the mold either has to be butted to another mold, or it has to wrap around a cylinder to meet itself. When a mold is joined to another, or if wrapped to join to itself, a seam line will appear. This seam line will show up on the reflective material manufactured from the prismatic mold. In some instances seam lines may be acceptable, and up until now they have had to be acceptable. In other instances seam lines are unacceptable especially in consumer reflector products. When making a large number of small consumer reflectors out of a large sheet of flexible prismatic materials, the product fabricating dies have had to work within the confines of the largest prismatic mold size such as 9xe2x80x3xc3x979xe2x80x3 or 9xe2x80x3xc3x9710xe2x80x3.
What is desired, therefore, is a way to eliminate the visually disturbing appearance of seams on retro-reflective materials made from prismatic manufacturing molds.
It is, therefore, an object of the present invention to provide prismatic reflective articles without visually disturbing seam lines and a method for making such articles.
Still another object of the invention is to provide a way to remove the visually disturbing presence of seam lines from such retro-reflective articles where multiple molds, used to make the articles, may need to be butted together.
It is yet another object of the invention to provide a way to hide a seam line from prismatic retro-reflective articles where one or more flexible metal or synthetic molds form a complete wrap around a cylinder for the manufacture of prismatic retro-reflective materials in a continuous manner.
Another object of the invention is to provide a way to completely remove seam lines from prismatic retro-reflective structures where one or more flexible molds have been wrapped around a cylinder in a method for continuous cylindrical manufacturing of the retro-reflective materials.
Yet another objective of the invention is to provide a way to remove visually disturbing seam lines from a retro-reflective material using economically small sized diamond-turned master molds and using electro formed duplicate molds in a tiled configuration to create larger master molds.
These objectives of this invention are achieved by utilizing either a meshing technique or a disguise technique used in the manufacture of the dies for forming prismatic retro-reflector articles in accordance with the invention.
In one embodiment in accordance with the invention a mold is formed in which prismatic elements are inscribed and where regularly spaced grid lines are added. These additional grid lines are separated by spacings that are so selected that an actual seam line formed by abutting molds would appear as a regular grid line and thus no longer appear as a visually disturbing seam line in a prismatic retro-reflective article made with the mold. Such seam hiding is particularly effective when micro fine prisms are used on the mold and enables the manufacture of continuous retro-reflective sheeting bearing prismatic retro-reflectors.
Hence, when making very fine precise prismatic tooling for retro-reflective cube-corner prisms, this invention teaches one how to hide the seam lines arising from joined mold structures, or occurring where mold edges meet each other.
In another embodiment in accordance with the invention a prismatic mold can be made having a fairly coarse prism structure, and wherein the prisms along an edge of the mold are arranged to mesh with those of a corresponding and abutting mold to effectively eliminate a visually disturbing seam in a retro-reflective sheet formed with the abutting molds.
The invention and its particular features will become more apparent from the following detailed description considered with reference to the accompanying drawings.